A big part of it is signaling a concern for the underprivileged/disabled/whatever. There's often an element of ideological projection. The thinking is, "Even if there's not a single deaf person in the audience, we're the type of organization that goes out of our way to be inclusive. So, if any deaf person is watching, they won't feel left out." In more pragmatic scenarios, sometimes an interpreter is requested specifically because deaf people will be in the audience. Does it make sense in sheer economic term? Is the value-added that great? Well, maybe. Hard to say. Because the social signaling I noted above itself has PR value and thus economic value.